Route signalling based resilient application overlay network

ABSTRACT

A system and method for handling critical events in service delivery gateways. Events are defined that cause a transition from a master redundancy state to a standby redundancy state in service delivery gateways and a plurality of signal-routes are stored. Each signal-route is associated with one or more of the defined events. A first defined event is detected in the first service delivery gateway and causes a transition from the first master redundancy state to the first standby redundancy state in the first service delivery gateway and a change in a first signal-route from the plurality of signal-routes in the first service delivery gateway. The change in the first signal-route is advertised and a second service delivery gateway transitions from the first standby redundancy state to the first master redundancy state.

This application claims the benefit of U.S. Provisional Application Nos. 62/141,240 and 62/141,249, both filed Mar. 31, 2015, both of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The techniques of this disclosure relate to computer networks and, more specifically, to providing high availability within computer networks.

BACKGROUND

A computer network is a collection of interconnected computing devices that can exchange data and share resources. In a packet-based network, the computing devices communicate data by dividing the data into small blocks called packets, which are individually routed across the network from a source device to a destination device. The destination device extracts the data from the packets and assembles the data into its original form. Dividing the data into packets enables the source device to resend only those individual packets that may be lost during transmission.

Certain devices, referred to as routers, maintain routing information that describes routes through the network. A “route” can generally be defined as a path between two locations on the network. Routers include a control plane, sometimes called a management plane, which maintains the routing information, and a forwarding plane, which forwards received packets according to the routing information.

The goal of high availability computer network environments is to provide users and other entities with “always on” service. That is, high availability computer network environments should provide reliable, continuous operation service. To accomplish this, network devices in a high availability environment perform error detection and implement recoverability for detected errors. Unfortunately, network devices occasionally fail.

When a network device fails, all network traffic flowing through the failed network device may cease. For an enterprise that depends on such network traffic, this may be unacceptable, even if this failure occurs only for a short time. To minimize the possibility of a failure causing all network traffic to cease, redundant hardware such as a standby controller or a separate standby network device may be installed. When the primary controller fails, this primary controller (which may also be referred to as a “master controller”) may switch over (or, in other words, fail-over) to the standby controller. Likewise, when the primary network device fails, this primary network device (which may also be referred to as a “master network device”) may switch over (or, in other words, fail-over) to the standby network device. After failing over or switching over to the standby device, the standby device becomes the master device.

Redundancy in devices or controllers that extends across two or more chassis provides enhanced reliability. Current inter-chassis redundancy solutions, however, are geared toward providing redundancy across two homogeneous chassis within the same network. A typical network, however, is not a collection of homogeneous chassis.

SUMMARY

In general, a framework is described for application aware inter-chassis redundancy with granular control to failover groups of applications between set of two or more network elements. The framework provided by the techniques may easily be used to define user interface constructions that leverage routing protocols for facilitating redundancy-related actions, such as redirecting traffic between service gateways. The network elements can be homogeneous or heterogeneous (physical or virtual) and can spread across different networks or across geographies. The redundancy mechanism provides traffic redirection agnostic of underlying network protocols and provides options for triggering, preventing and resuming both manual and automated switchovers of groups of services based on their health status.

For example, a method is described that includes receiving configuration information defining events that cause a transition from a master redundancy state to a standby redundancy state in service delivery gateways, including receiving configuration information defining a first event that causes a transition from a first master redundancy state to a first standby redundancy state in service delivery gateways; in response to receiving the configuration information, storing a plurality of signal-routes, including a first signal-route, wherein each signal-route is associated with one or more of the defined events; and in response to detecting occurrence of the first event in a first service delivery gateway: transitioning, within the first service delivery gateway, from the first master redundancy state to the first standby redundancy state; making a change in a set of available signal-routes stored in the first service delivery gateway, wherein making a change in a set of available signal-routes includes adding the first signal-route to or removing the first signal-route from the set of available signal routes in the first service delivery gateway; and advertising, from the first service delivery gateway and to peer network devices, the change in the first signal-route.

According to another example, a service delivery gateway system includes a plurality of links and a plurality of service delivery gateway network devices connected by the plurality of links. The plurality of service delivery gateway network devices includes a first service delivery gateway network device and a second service delivery gateway network device, wherein each service delivery gateway network device includes a memory and one or more processors connected to the memory, wherein the one or more processors are configured to: receive configuration information defining events that cause a transition from a master redundancy state to a standby redundancy state in service delivery gateway network devices, including receiving configuration information defining a first event that causes a transition from a first master redundancy state to a first standby redundancy state in service delivery gateway network devices; in response to receiving the configuration information, store a plurality of signal-routes, including a first signal-route, wherein each signal-route is associated with one or more of the defined events; and in response to detecting occurrence of the first event in the first service delivery gateway network device: transition, within the first service delivery gateway network device, from the first master redundancy state to the first standby redundancy state; transition, within the second service delivery gateway, from the first standby redundancy state to the first master redundancy state; make a change in a set of available signal-routes stored in the first service delivery gateway, wherein making a change in a set of available signal-routes includes adding the first signal-route to or removing the first signal-route from the set of available signal routes in the first service delivery gateway; and advertise, from the first service delivery gateway and to peer network devices, the change in the first signal-route.

According to yet another example, a computer-readable storage medium stores instructions that, when executed, cause one or more processors to: receive configuration information defining events that cause a transition from a master redundancy state to a standby redundancy state in service delivery gateway network devices, including receiving configuration information defining a first event that causes a transition from a first master redundancy state to a first standby redundancy state in service delivery gateway network devices; in response to receiving the configuration information, store a plurality of signal-routes, including a first signal-route, wherein each signal-route is associated with one or more of the defined events; and in response to detecting occurrence of the first event in a first service delivery gateway network device: transition, within the first service delivery gateway network device, from the first master redundancy state to the first standby redundancy state; make a change in a set of available signal-routes stored in the first service delivery gateway, wherein making a change in a set of available signal-routes includes adding the first signal-route to or removing the first signal-route from the set of available signal routes in the first service delivery gateway; and advertise, from the first service delivery gateway and to peer network devices, the change in the first signal-route.

The details of one or more embodiments of the techniques are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example redundant service delivery gateway system operating in accordance with techniques described herein.

FIG. 2 illustrates redundancy groups and redundancy sets on service delivery gateways.

FIG. 3 is a block diagram illustrating an example service delivery gateway in accordance with the techniques described in this disclosure.

FIG. 4 is a block diagram illustrating an example set of service chains of services according to the techniques described herein.

FIG. 5 is a block diagram illustrating master and standby relationships across service delivery gateways.

FIG. 6 is a block diagram illustrating communication between gateways in the network system of FIG. 1.

FIG. 7 is a block diagram illustrating a mastership transition to a peer in accordance with techniques described herein.

FIG. 8A is a block diagram illustrating communication between application nodes in the redundant service delivery gateway system of FIG. 1.

FIG. 8B is a representative route-signal vector in accordance with techniques described herein.

FIG. 9 is a diagram illustrating advertising the presence or absence of a signal-route through the use of an as-path-prepend command.

FIG. 10 is a diagram illustrating advertising the presence or absence of a signal-route through the use of local-preference values.

FIG. 11 is a flowchart illustrating services switchover to a peer in accordance with techniques described herein.

FIG. 12 is a block diagram illustrating a redundancy set state machine for moving between master and standby states in accordance to the techniques described herein.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an example redundant service delivery gateway system 4 operating in accordance with techniques described herein. In the example approach shown in FIG. 1, redundant service delivery gateway system 4 includes two or more service delivery gateways (here, gateways 8A and 8B and, collectively, “gateways 8”) logically associated as a redundant service delivery system 27 in accordance with one or more aspects of the techniques described in this disclosure in order to provide high availability. In one example approach, redundant service delivery gateway system 4 of FIG. 1 includes a subscriber access network 6 (“access network 6”) connected to a service provider core network 7 and, through service provider core network 7, to public network 12. In one example approach, service provider core network 7 operates as a private network to provide packet-based network services to subscriber devices 16A-16N (collectively, “subscriber devices 16”) across access network 6. In one such example approach, service provider core network 7 provides authentication and establishment of network access for subscriber devices 16 such that the subscriber device may begin exchanging data packets with public network 12, which may be an internal or external packet-based network such as the Internet.

In the example of FIG. 1, access network 6 provides connectivity to public network 12 via service provider core network 7 and gateways 8. In one example approach, service provider core network 7 and public network 12 provide packet-based services that are available for request and use by subscriber devices 16. As examples, core network 7 and/or public network 12 may provide, for example, bulk data delivery, voice over Internet protocol (VoIP), Internet Protocol television (IPTV), Short Messaging Service (SMS), Wireless Application Protocol (WAP) service, or customer-specific application services. Public network 12 may include, for instance, a local area network (LAN), a wide area network (WAN), the Internet, a virtual LAN (VLAN), an enterprise LAN, a layer 3 virtual private network (VPN), an Internet Protocol (IP) intranet operated by the service provider that operates access network 6, an enterprise IP network, or some combination thereof. In various example approaches, public network 12 is connected to a public WAN, the Internet, or to other networks. In some such examples, public network 12 executes one or more packet data protocols (PDPs), such as IP (IPv4 and/or IPv6), X.25 or Point-to-Point Protocol (PPP), to enable packet-based transport of public network 12 services.

As shown in the example of FIG. 1, each of the service delivery gateways 8 provide a set of services 10. In some example approaches, gateways 8 provide these services via a service plane within each of gateways 8. Subscriber devices 16 connect to gateways 8 via access network 6 to receive connectivity to subscriber services for applications hosted by subscriber devices 16. A subscriber may represent, for instance, an enterprise, a residential subscriber, or a mobile subscriber. Subscriber devices 16 may be, for example, personal computers, laptop computers or other types of computing device associated with subscribers. In addition, subscriber devices 16 may comprise mobile devices that access the data services of redundant service delivery gateway system 2 via radio access network (RAN) 9. Example mobile subscriber devices include mobile telephones, laptop or desktop computers having, e.g., a 3G wireless card, wireless-capable netbooks, video game devices, pagers, smart phones, personal data assistants (PDAs) or the like. Each of subscriber devices 16 may run a variety of software applications, such as word processing and other office support software, web browsing software, software to support voice calls, video games, videoconferencing, and email, among others. In some example approaches, subscriber devices 16 connect to access network 6 via access links 5 that comprise wired and/or wireless communication links. The term “communication link,” as used herein, comprises any form of transport medium, wired or wireless, and can include intermediate nodes such as network devices. Each of access links 5 may comprise, for instance, aspects of an asymmetric DSL network, WiMAX, a T-1 line, an Integrated Service Digital Network (ISDN), wired Ethernet, or a cellular radio link.

In some example approaches, a network service provider operates, or in some cases leases, elements of access network 6 to provide packet transport between subscriber devices 16 and gateways 8. Access network 6 represents a network that aggregates data traffic from one or more subscriber devices 16 for transport to/from service provider core network 7 of the service provider. In some example approaches, access network 6 includes network nodes that execute communication protocols to transport control and user data to facilitate communication between subscriber devices 16 and gateways 8. Access network 6 may include a broadband access network, network, a wireless LAN, a public switched telephone network (PSTN), or other type of access network, and may include or otherwise provide connectivity for cellular access networks, such as radio access network (RAN) 9 of FIG. 1. Examples of radio access network 9 include networks conforming to a Universal Mobile Telecommunications System (UMTS) architecture, an evolution of UMTS referred to as Long Term Evolution (LTE), mobile IP standardized by the Internet Engineering Task Force (IETF), as well as other standards proposed by the 3^(rd) Generation Partnership Project (3GPP), 3^(rd) Generation Partnership Project 2 (3GGP/2) and the Worldwide Interoperability for Microwave Access (WiMAX) forum.

Service provider core network 7 (hereinafter, “core network 7”) offers packet-based connectivity to subscriber devices 16 attached to access network 6 for accessing public network 12. Core network 7 may represent a public network that is owned and operated by a service provider to interconnect a plurality of networks, which may include access network 6. Core network 7 may implement Multi-Protocol Label Switching (MPLS) forwarding and in such instances may be referred to as an MPLS network or MPLS backbone. In some instances, core network 7 represents a plurality of interconnected autonomous systems, such as the Internet, that offers services from one or more service providers. Public network 12 may represent an edge network coupled to core network 7, e.g., by a customer edge device such as customer edge switch or router. Public network 12 may include a data center.

In examples of service delivery gateway system 4 that include a wireline/broadband access network such as subscriber access network 6, each of gateways 8 may represent a Broadband Network Gateway (BNG), a Broadband Remote Access Server (BRAS), MPLS Provider Edge (PE) router, core router or gateway, or a Cable Modem Termination System (CMTS), for instance. In examples of service delivery gateway system 4 that include a cellular access network such as subscriber access network 6, each of gateways 8 may represent a mobile gateway, for example, a Gateway General Packet Radio Service (GPRS) Serving Node (GGSN), an Access Gateway (aGW), or a Packet Data Network (PDN) Gateway (PGW). In other examples, the functionality described with respect to each gateway 8 may be implemented in a switch, service card or other network element or component.

A network service provider administers at least parts of service delivery gateway system 4, typically offering services to subscribers associated with devices, e.g., subscriber devices 16, that access service delivery gateway system 4. Services offered may include, for example, traditional Internet access, Voice-over-Internet Protocol (VoIP), video and multimedia services, and security services. As described above with respect to access network 6, service provider core network 7 may support multiple types of access network 6 infrastructures that connect to service provider network access gateways to provide access to the offered services. In some instances, a service delivery gateway system 4 may include subscriber devices 16 that attach to multiple different access networks 6 having varying architectures.

In general, applications executing on one or more of subscriber devices 16 may request authorization and data services by sending a session request to one or more of service delivery gateways 8. In turn, service delivery gateways 8 typically accesses an Authentication, Authorization and Accounting (AAA) server 11 to authenticate the subscriber device requesting network access. In some examples, service delivery gateways 8 query policy control server 14 and/or AAA server 11 to determine subscriber-specific service requirements for packet flows from subscriber devices 16.

Once authenticated, any of subscriber devices 16 may send subscriber data traffic toward service provider core network 7 in order to access and receive services provided by public network 12. Such packets traverse service delivery gateways 8 as part of at least one packet flow. The term “packet flow,” “traffic flow,” or simply “flow” refers to a set of packets originating from a particular source device and sent to a particular destination device. A single flow of packets, in either the upstream (sourced by one of subscriber devices 16) or downstream (destined for one of subscriber devices 16) direction, may be identified by, for example, the 5-tuple: <source network address, destination network address, source port, destination port, protocol>. This 5-tuple generally identifies a packet flow to which a received packet corresponds. An n-tuple refers to any n items drawn from the 5-tuple. For example, a 2-tuple for a packet may refer to the combination of <source network address, destination network address> or <source network address, source port> for the packet. Moreover, a subscriber device may originate multiple packet flows upon authenticating to service provider core network 7 and establishing a communication session for receiving data services. Path 26 illustrates routing of data from subscriber devices to public network 12 and back as defined by one or more gateways 8.

As described herein, service delivery gateways 8 provide services 10 to some or all of the network traffic. As examples, services 10 in one or more gateways 8 may apply firewall and security services, network address translation (NAT) or carrier grade network address translation (CG-NAT), media optimization (voice/video), IPSec/VPN services, deep packet inspection (DPI), session border controller (SBC), virtual appliance, virtual cache, network traffic acceleration, Quality of Service (QoS), access control, hyper-text transfer protocol (HTTP) filtering, counting, accounting, charging, and load balancing of packet flows or other types of services applied to network traffic. In some examples, services provided by service delivery gateways 8 may be composite services composed of two or more services, and may form a single externally visible service to subscribers 16. As one example, services 10 may be a composite service consisting of NAT services and firewall services.

In some examples, each of services 10 may run as virtual machines in a virtual compute environment provided by service delivery gateways 8 or within other execution environments. For example, although described herein as provided by compute blades within service delivery gateways 8, the compute environment for services 10 may instead, or in addition, be provided by a scalable cluster of general computing devices, such as x86 processor-based servers. As another example, services 10 may reside on a combination of general purpose computing devices and special purpose appliances. Service 10 may also be virtualized, individual network services that scale as in a modern data center, through the allocation of virtualized memory, processor utilization, storage and network policies, as well as by adding additional load-balanced virtual machines.

In one example approach, gateways 8 steer individual subscriber packet flows through defined sets of services provided by services 10. That is, each subscriber packet flow may be forwarded through a particular ordered combination of services provided by services 10 within particular gateways 8, each ordered set being referred to herein as a “service chain.” Moreover, a given service chain may include network services provided “on box” within service deliver gateways 8 or “off box” by a separate computing environment accessed by the gateways 8, or by combinations thereof. In this way, subscriber flows may be processed by gateways 8 as the packets flow between access network 6 and public network 12 according to service chains configured by the service provider. Some techniques for accomplishing this are described in U.S. patent application Ser. No. 14/042,685, entitled “Session-Aware Service Chaining Within Computer Networks,” filed Sep. 30, 2013, the descriptions of which are incorporated herein by reference.

Once processed at a terminal node of the service chain, i.e., the last service applied to packets flowing along a particular service path, the terminal node may direct the traffic back to the forwarding plane of gateway 8 for further processing and/or for forwarding to public network 12.

Whereas a “service chain” defines one or more services to be applied in a particular order to provide a composite service for application to packet flows bound to the service chain, a “service tunnel” or “service path” refers to a logical and/or physical path taken by packet flows processed by a service chain along with the forwarding state for forwarding packet flows according to the service chain ordering. Each service chain may be associated with a respective service tunnel, and packet flows associated with each subscriber device 16 flow along service tunnels in accordance with a service profile associated with the respective subscriber. Gateways 8, after authenticating and establishing access sessions for the subscribers, may determine that a profile of each subscriber device 16 requires the traffic to be sent on a service tunnel to one or more service nodes 13 for application of services, and directs packet flows for the subscribers along the appropriate service tunnels within each gateway 8, thereby causing services 10 (e.g., service nodes 13 that provide the services) to apply the requisite ordered services for the given subscriber.

Services 10 may, for instance, represent one or more service nodes that implement service chains using internally configured forwarding state that directs packets of the packet flow along the service chains for processing according to the identified set of services 10. Such forwarding state may specify tunnel interfaces for tunneling between services 10 using network tunnels such as Internet Protocol (IP) or Generic Route Encapsulation (GRE) tunnels, or by using Virtual Local Area Networks (VLANs), Multiprotocol Label Switching (MPLS) techniques, and so forth. In some instances, real or virtual switches, routers or other network elements that interconnect services 10 may be configured to direct packet flow to services 10 according to the service chains.

As noted above, redundancy in devices or controllers that extend across two or more chassis provides enhanced reliability. Current inter-chassis redundancy solutions, however, are geared toward providing redundancy across two homogeneous chassis within the same network. A typical network, however, is not a collection of homogeneous chassis. To compensate, as described herein, service delivery gateways 8 may provide user interfaces programmed to support semantics and commands that allow a user to more efficiently define and administer active-active or active-standby redundancy with respect to services 10 applied to packet flows within service delivery gateway system 4. In one example approach, the user interface allows an administrator or network management system to easily specify configuration data defining redundancy mechanisms for administering a cluster of redundant service gateways. The techniques described herein decouple application-layer redundancy mechanisms from underlying communication mechanisms between the devices, thereby allowing protocols, such as routing protocols and inter-chassis communication protocols, to easily be leveraged by the abstracted redundancy mechanisms. The techniques of this disclosure can also allow an administrator to easily configure redundancy arrangements on gateways 8 on a per-service and/or per composite service level of granularity.

Moreover, the techniques of this disclosure provide a management interface expressivity, i.e., a syntax, that leverages an abstraction that can be used across different types of gateways to hide the underlying hardware. This management interface expressivity may be more useful for system administrators, and may also drive the behavior of service provider core network 7 and service delivery gateways 8 more efficiently.

In this way, in one example, the framework provided by the techniques may easily be used to define user interface constructions that leverage routing protocols for facilitating redundancy-related actions, such as causing network devices of service provider core network 7 to redirect traffic from one of service delivery gateways 8 to the other. For example, as described herein, the user interface constructs may be defined to configure the application layer services to easily add or remove so-called “signal-routes” to trigger actions related to the redundancy mechanisms, such as groups of redundant devices or routing/switching instances. In one example approach, a signal-route is a route used by applications using the services redundancy process described below to signal changes in application mastership state and drive routing-policy changes at the same time. In one such example approach, “signal-routes” are static routes manipulated by the service delivery gateway to affect the routing-policies in order to switch mastership between redundant service delivery gateways and to redirect traffic to the new master service delivery gateway.

In one example, the techniques described herein provide user interface (UI) building blocks that allow the user to define and specify logical constructs for a redundant service delivery system, such as redundant service delivery system 27 of FIG. 1. The UI building blocks include a support for a syntax allowing a user to define critical events that trigger a switch from gateway mastership to a standby state (“Redundancy Events”). In one example approach, a Redundancy Event (RE) is an event that triggers a services redundancy (SR) daemon to switch gateway mastership from one service delivery gateway 8 to another. For example, the user may define a redundancy event in terms of a degradation of performance of the service 10 such that a service node can no longer provide the services paid for by a service level agreement (SLA) associated with one of subscriber devices 16.

In one example approach, the UI building blocks include support for a syntax allowing a user to define a policy (“Redundancy Policy (RP)”) that defines how redundancy events are tied to actions to be taken on the occurrence of the 1 events defined by the redundancy events. In some such approaches, a redundancy policy is a policy that details the actions to be taken on the occurrence of one or more of the underlying critical events. In some examples, the actions may specify that a service redundancy process of service delivery gateway 8A updates routing information maintained by the service delivery gateway 8A, prompting a routing protocol executing on service delivery gateway 8A to issue a routing protocol update to routing peers (not shown) within service provider core network 7.

In one example approach, the UI building blocks also include support for a syntax allowing a user to group one or more redundancy policies into a set, termed a “Redundancy Set (RS),” and a a syntax allowing a user to group one or more redundancy sets into a group of sets, termed a “Redundancy Group (RG).” In this manner, service delivery gateways 8 include respective user interfaces that support a syntax that allows a user to define one or more redundancy events, redundancy policies, redundancy sets, and redundancy groups, as explained in further detail below. The ability to create redundancy sets and redundancy groups can allow for defining multiple redundancy groups across the same set of service delivery gateway chassis, for example.

In accordance with the techniques of this disclosure, a services redundancy process of each of service delivery gateways 8 monitors performance levels of services 10. In the event that the monitor component detects a failure or degeneration of pre-set service levels for any of services 10 that meets the definition of a redundancy event, the services redundancy process triggers application of a pre-defined redundancy policy. In some aspects, for example, the services redundancy process may interact with services 10 to collect statistics, perform handshaking, or carry out other checks to the functionality of services 10.

The performance level of the services 10 are independent of an overall operational state of the service delivery gateway network devices 8. In other words, upon detecting a configured redundancy event, the service delivery gateway 8 is able to trigger redundancy-related actions for the service 10. This may include, for example, changing primary/standby roles associated with a redundancy set or redundancy group from service delivery gateway 8A to service delivery gateway 8B, for example, even though service delivery gateway 8A and/or the existing service node used for application of the affected service 10 remains operable. The switchover of network traffic requiring the services 10 occurs without disruption to a subscriber 16 receiving the services 10.

FIG. 2 is a block diagram illustrating example redundancy groups and redundancy sets that may be defined on service delivery gateways 8 using the syntax supported by the user interface as described herein. In the example shown in FIG. 2, service delivery gateways 8A, 8B and 8C are connected in a redundant service delivery system 27 controlled by one or more service redundancy (SR) daemons 24. UI building blocks are used to define events (Redundancy Events), to define a redundancy policy for reacting to such events (Redundancy Policies), and to group the redundancy policies into sets (Redundancy Sets). The redundancy policies detail the action to take on the occurrence of the defined redundancy event.

In one example approach, a redundancy set not only groups one or more redundancy policies into a set, but also assigns states to that set. In one such approach, each redundancy set includes a master state and at least one standby state; the UI building blocks include a technique for defining the critical events that lead to a change in state. In one example approach, each redundancy set therefore establishes the granularity of conditions that drive changes in master/standby states as a function of redundancy policies and their underlying redundancy events. In one example approach, a redundancy set also binds one or more service-sets to drive the Stateful Synchronization state related to these service sets, in which state is synchronized across service delivery gateways based on the redundancy sets for potential failover of redundancy sets.

As noted above, in one example approach, the UI building blocks described herein include a technique for grouping two or more redundancy sets into a “Redundancy Group (RG).” In one such example approach, a redundancy group 22 is a collection of one or more redundancy sets 20; redundancy groups are defined for each service delivery gateway 8. In the example shown in FIG. 2, redundant service delivery system 27 includes three application aware redundancy sets 20 (RS1-RS3) across three redundancy groups 22 (RG1-RG3) between three Service Delivery Gateways 8 (SDG1-SDG3). In the example shown in FIG. 2, service delivery gateway 8A (SDG1) includes two redundancy groups 22, RG1 and RG2. RG1 of SDG1 includes a single redundancy group 22 (RS1), while RG1 of SDG3 includes RS1 and RS2.

The UI framework defined herein provides the ability to extend service redundancy across chassis for different groups, events and actions. The framework allows administrators to define custom events that can be used as triggers for switchovers and custom redundancy polices that include actions to be taken for the switchovers. The chassis that make up the redundancy groups can be homogeneous or heterogeneous chassis, can be connected over L2 or L3 networks, and can be geographically separated.

FIG. 3 is a block diagram illustrating an example service delivery gateway network device in accordance with the techniques described in this disclosure. In the example of FIG. 3, the service delivery gateway network device (service delivery gateway 8) includes a forwarding plane 130, a routing plane 132 and a service plane 134. Forwarding plane 130 may be provided by dedicated forwarding integrated circuits normally associated with high-end routing and forwarding components of a network router. U.S. Pat. No. 8,050,559, issued Nov. 1, 2011 and entitled MULTI-CHASSIS ROUTER WITH MULTIPLEXED OPTICAL INTERCONNECTS, describes a multi-chassis router in which a multi-stage switch fabric, such as a 3-stage Clos switch fabric, is used as a high-end forwarding plane to relay packets between multiple routing nodes of the multi-chassis router, the descriptions of which are incorporated herein by reference.

Service delivery gateway 8 may integrate a routing plane 132 and a service plane 134 in a manner that utilizes shared forwarding plane 130. Forwarding plane 130 may represent a rich and dynamic shared forwarding plane, in some cases distributed over a multi-chassis router. Moreover, forwarding plane 130 may be, as noted above, provided by dedicated forwarding integrated circuits normally associated with high-end routing components of a network router such that routing plane 132 and forwarding plane 130 operate as a high-end router. In one example approach, service plane 134 may be tightly integrated within service delivery gateway 8 (e.g., by way of service cards 136) so as to use forwarding plane 130 of the routing components in a shared, cooperative manner. Details of such routing can be found in U.S. Pat. No. 8,339,959, issued Dec. 25, 2012 and entitled “STREAMLINED PACKET FORWARDING USING DYNAMIC FILTERS FOR ROUTING AND SECURITY IN A SHARED FORWARDING PLANE,” the descriptions of which are incorporated herein by reference.

As seen in FIG. 3, routing plane 132 provides a routing component 138 that is primarily responsible for maintaining a routing information base (RIB) 140 to reflect the current topology of a network and other network entities to which service delivery gateway 8 is connected. For example, routing component 138 provides an operating environment for execution of routing protocols by a routing protocol process such as routing protocol daemon 142 (RPd). Example protocols include routing and label switching protocols, such as a border gateway protocol (BGP), Open Shortest Path First (OSPF), intermediate-systems to intermediate-system (ISIS) routing protocol, a resource reservation protocol (RSVP), RSVP with traffic engineering extensions (RSVP-TE), an interior gateway protocol (IGP), link state protocols, and a label distribution protocol (LDP). Routing protocol daemon 142 may represent a software component or module that communicates with peer routers and periodically updates RIB 140 to accurately reflect the topology of the network and the other network entities. While described as a daemon or software module executed by routing engine 44, routing daemon 61 may be implemented as a hardware module or as a combination of both hardware and software.

Routing component 138 may receive this routing information via routing protocol daemon 142 and update or otherwise maintain RIB 140 to reflect a current topology of core network 7. This topology may provide for multiple different paths through core network 7 to reach any given subscriber device 16. In the example of FIG. 1, a path exists from public network 12 through each of service delivery gateways 8 to subscriber devices 16. Routing component 138 in one of the gateways 8 may, in some instances, select the path to use to connect a subscriber device 16 to public network 12.

In the example shown in FIG. 3, admin 145 may interface with routing component 138 via a user interface (UI) module 146, which may represent a module by which a user or provisioning system may interface with routing component 138. UI module 146 may, for example, include a command line interface (CLI), which may accept inputs in the form of commands and/or scripts, or may include a graphical user interface (GUI). Admin 145 may interface with UI module 146 to configure various components service delivery gateway 8, including routing component 138. Once configured, routing component 138 may then resolve RIB 140 to generate forwarding information. Routing component 138 may then interface with forwarding plane 130 to install this forwarding information into a forwarding information base (FIB) 148.

Ingress forwarding component 150A and egress forwarding component 150B (“forwarding components 150”) may represent software and/or hardware components, such as one or more interface cards (not shown), that forward network traffic. In one example approach, forwarding component 150A maintains FIB 148 that associates network destinations or with specific next hops and corresponding interface ports of output interface cards of service delivery gateway 8. In some such example approaches, routing component 138 generates FIB 148 in the form of a radix tree having leaf nodes that represent destinations within network 7. U.S. Pat. No. 7,184,437, issued Feb. 27, 2007, provides details on exemplary example approaches of a router that utilizes a radix tree for route resolution, the descriptions of which are incorporated herein by reference.

In one such example approach, when forwarding a packet, forwarding component 150A traverses the radix tree to a leaf node based on information within a header of the packet to ultimately select a next hop and output interface to which to forward the packet. Based on the selection, forwarding component may output the packet directly to the output interface or, in the case of a multi-stage switch fabric of a high-end router, may forward the packet to subsequent stages for switching to the proper output interface.

As seen in FIG. 3, service plane 134 represents a logical or physical plane that provides one or more services using service cards 136. Service cards 136A and 136B (collectively “service cards 136”) may represent physical cards that are configured to be inserted into service delivery gateway 8 and coupled to forwarding plane 130 and routing plane 132 via a backplane, switch fabric or other communication medium. Typically, service cards 136 may comprise cards that couple directly to the switch fabric. Service cards 136 may be removable from service delivery gateway 8. Admin 145 may interface with UI module 146 to interface with routing component 138 to specify which packet flows are to undergo service processing by one or more of service cards 136. After specifying these flows, routing component 138 may update RIB 140 to reflect that these flows are to undergo service processing, such that when resolving FIB 148, the forwarding information may indicate that various flows are to undergo service processing. Often, this forwarding information may specify that these flows require service processing by specifying a next hop for these flows that directs packets of these flows to one of service cards 136 (where this next hop may be referred to as an “internal next hop”), as described in further detail below. Additional next hops may be specified that are external to service delivery gateway 8, where the external next hop may specify, in this example, on which path the packet is to be forwarded. The internal next hop may be linked to the external next hop, where in this example, service delivery gateway 8 may maintain two next hops (and possibly more) for any given flow.

Service cards 136 may each represent a card capable of applying one or more services. Service card 136 may include a control unit 151, which may represent one or more general processors that execute software instructions, such as those used to define a software or computer program, stored to a non-transitory computer-readable medium such as a storage device (e.g., a disk drive, or an optical drive), or memory (such as Flash memory, random access memory or RAM) or any other type of volatile or non-volatile memory, that stores instructions to cause the one or more processors to perform the techniques described herein. Alternatively, control unit 151 may represent dedicated hardware, such as one or more integrated circuits, one or more Application Specific Integrated Circuits (ASICs), one or more Application Specific Special Processors (ASSPs), one or more Field Programmable Gate Arrays (FPGAs), or any combination of one or more of the foregoing examples of dedicated hardware, for performing the techniques described herein. In some instances, control unit 151 may be referred to as a processor.

Control unit 151 may implement a service engine 152, which may represent a module or unit that applies one or more services to packets, flows of packets and/or sessions of packets (where a session refers to the combination of a flow to a destination from a source and a flow from the same destination to the same source). Service engine 152 may be a software and/or hardware component that applies services in accordance with service policy rules defined by policy configuration data stored by service policies (not shown). The service policies may be configured by administrator 145 via UI module 146, and programmed by management daemon 160, for example. Service engine 152 may perform any type of service, including those listed above and below. For purposes of illustration, it is assumed that service engine 152 implements a service that modifies, edits or updates information in packets that is generally used in performing path selection or otherwise making forwarding decisions. Example services that modify, edit or updates this information may comprise a NAT service and a tunneling service.

In the example of FIG. 3, forwarding component 150A receives a packet 149 and, acting as an ingress forwarding component, invokes a flow control unit 154. Flow control unit 154 represents a module that selectively directs packets to service plane 134 for processing. In some example approaches, service-plane 134 is a virtual machine. Flow control unit 154 may access FIB 148 to determine whether packet 149 is to be sent to an internal next hop, e.g., one of service cards 136 of service plane 134, or to an external next hop via another one of forwarding components that acts as an egress forwarding component for the flow to which packet 149 corresponds, such as, e.g., egress forwarding component 150B. While referred to as ingress forwarding component 150A and egress forwarding component 150B, each of forwarding components 150A, 150B may be the same or similar to one another in terms of underlying hardware and/or logic. That is, the ingress and egress designation of forwarding components 150A, 150B is merely to denote that forwarding component 150A acts as the ingress forwarding component for the packet flow to which packet 149 corresponds and forwarding component 150B acts as the egress forwarding component for the packet flow to which packet 149 corresponds. Moreover, forwarding plane 130 may include more than two forwarding components, where these additional forwarding components are not shown in the example of FIG. 3 for ease of illustration purposes.

In any event, flow control unit 154 may determine that packet 149 is to be transmitted to service card 136. In response to determining that packet 149 is to be transmitted to service card 136 so that service card 136 can apply a service to packet 149, in some examples flow control unit 154 of ingress forwarding component 150A may append an internal service packet header (which may also be referred to as a “service cookie”). Flow control unit 154 may specify this internal service packet header to include a field that stores an ingress identifier that identifies forwarding component 150A. Flow control unit 154 may append this internal service packet header to packet 149 to generate an updated packet 156. Flow control unit 154 may then direct packet 156 to service card 136 of service plane 134. Service card 136 may receive this packet remove the internal service packet header, parsing the ingress identifier from the internal service packet header. Control unit 151 of service card 136 may then invoke service engine 152, which applies the service to updated packet 156, generating a serviced packet 158. Serviced packet 158 is assumed to differ from packet 149 and 156 in that at least one aspect of serviced packet 158 used when making forwarding decisions or performing path selection differs that of packets 149 and 156 (such as at least one aspect of the five-tuple of serviced packet 158 differs from the five-tuple of packet 149 and packet 156). In this respect, service card 136 applies the service to updated packet 156 to generate serviced packet 158 such that five tuple of serviced packet 158 is different from the five-tuple of updated packet 156.

Service card 136 may then transmit serviced packet 158 back to flow control unit 154 using the ingress identifier previously parsed from the internal service packet header so as to maintain load balancing of packet flows across forwarding components of service delivery gateway 8. That is, service card 136 may actively identify the one of forwarding components 150A, 150B (and any other forwarding components not shown in the example of FIG. 3 for ease of illustration purposes) that originally received packet 149, that acts as the so-called ingress forwarding component, and/or that maintains the single point of contact for the flow to which packet 149 corresponds. As a result, service card 136 transmits serviced packet 158 to ingress forwarding component 150A identified by the ingress identifier without applying a hash function to at least a portion of serviced packet 158 to identify ingress forwarding component 150A and/or without determining a next hop of the to which to forward serviced packet 158. Moreover, service card 136 transmits serviced packet 158 to ingress forwarding component 150A such that ingress forwarding component 150A receives the packet as if the packet had been received by ingress forwarding component 150A via an interface (not shown in the example of FIG. 3) associated with ingress forwarding component 150A that couples to another network device rather than via a switch fabric coupling service card 136 to ingress forwarding component 150A. By selecting ingress forwarding component 150A, service card 136 maintains the load balancing of packet flows across the links/forwarding components (of the receiving router) decided by the upstream router in accordance with weighted equal cost multi-path (WECMP).

Flow control unit 154 receives this serviced packet 158 and accesses FIB 148 using the five-tuple of serviced packet 158 in order to retrieve an entry specifying a next hop for the flow to which serviced packet 158 corresponds. In other words, flow control unit 154 determines the next hop to which to forward serviced packet 158 based on the five-tuple of serviced packet 158. Assuming flow control unit 154 identifies a next hop that involves forwarding serviced packet 158 via an interface associated with egress forwarding component 150B, flow control unit 154 forwards this packet 158 to egress forwarding component 150B, which in turn forwards packet 158 to the next hop.

As can be seen in FIG. 3, routing plane 132 includes a management daemon 160 coupled to user interface module 146 and to a configuration database 162. Management daemon 160 receives configuration information from user interface module 146 and stores the configuration information in configuration database 162. In some examples, routing plane 132 also includes a services redundancy daemon (SRd) 164 (also referred to herein as a services redundancy process), which operates in conjunction with routing policies database 166 to configure and control redundant services delivery system 27. SRd 164 also interfaces with service plane 134, such as to permit configuration of service cards 136A, 136B by management daemon 160. SRd 164 may represent a software module that updates RIB 140 based on configuration database 162 and route policies 166. While described as a daemon, software process, or software module executed by routing component 138, SRd 164 may be implemented as a hardware module or a combination of both hardware and software.

User interface module 146 may represent a software and/or hardware module that presents an interface with which an administrator or an administrative device, represented by “ADMIN” 145, may interact to specify certain operational characteristics of services redundancy gateway network device 8. In response to invocation by admin 145, user interface module 146 interacts with other components of services redundancy gateway network device 8, such as to retrieve, configure, copy, and/or delete policy configuration data stored in route policies 166, update service data of services plane 143 via SRd 164, and to perform other management-related functions. In one example approach, admin 145 may interact with user interface module 146 to enter configuration information for SRd 164, such as configuration information defining redundancy events, redundancy policies, redundancy sets and redundancy groups, and this configuration information is also stored in configuration database 162.

Redundancy sets will be discussed next. In one example approach, each redundancy set 30 includes a master state and one or more standby states. States get elected based, e.g., on the health of applications. During operation of service deliver gateway 8, one or more services redundancy processes (such as service redundancy daemons 164) in each service delivery gateway 8 may continuously monitor the health-status of the groups of applications and exchange this information across all the related chassis. For example, SRD 164 may detect a failure or degradation in an application (e.g., a service provided by a service engine 152 of service card 136A, such as a firewall service), and may notify Inter-Chassis Control Protocol (ICCP) module 155, which sends information about the affected application to a respective counterpart ICCP module executing on one or more other chassis.

SRd 164 may monitor performance of one or more of service engines 152 in service plane 134. Service plane 46 may provide an operating environment for running one or more applications. In some aspects, service engines 152 may each expose an application programming interface (API) by which SRd 164 inspects performance data (e.g., loading levels) for the respective service. Alternatively, SRd 164 may expose a universal interface, which each of service engines 152 may invoke to communicate current performance data. As another example, SRd 164 may periodically ping each service engine 152 or may monitor output communications from each of the services or operating-system level resources consumed by each of the services of service cards 136A, 136B. In some examples, SRd 164 can monitor any of a variety of parameters associated with service engines 152, which may be defined via a management plane of services delivery gateway network device 8, e.g., via user interface module 146 and management daemon 160. SRd 164 can monitor parameters associated with service engines 152 such as per-process central processing unit (CPU) usage, memory usage, rate of output, number of available connections, or any other parameters for detecting whether a service 52 is performing according to expected levels. For example, if a service 52 is expected to provide an output at a threshold rate, SRd 164 can detect when the actual rate of output falls below the threshold rate. An administrator 145 can configure the performance level thresholds via user interface module 146, such as when an application or other service is initially deployed on services delivery gateway network device 8. The performance level thresholds may be stored in configuration database 162. The performance level thresholds may be selected relative to SLA requirements, to trigger action when performance levels fall below what is required by subscribers 16, for example.

SRD 164 may also continuously monitor for system events of gateway 8, such as interface down events, physical interface card (PIC) reboots, flexible PIC concentrator (FPC) reboots, RPD aborts/restarts, and peer gateway events. For example, SRd 164 may communicate with Bidirectional Forwarding Detection (BFD) module 157 and/or ICCP module 155 in forwarding plane 130 to obtain information by which to detect occurrence of system events.

On detecting the occurrence of redundancy events including application-related events or critical system events, depending on its mastership state, in some example approaches, SRD 164 communicates with the network layer in a protocol agnostic manner to redirect traffic to the next standby node that gets elected as the master. For example, in response to SRD 164 detecting a redundancy event and in accordance with route policies 166 previously defined by administrator 145, SRD 164 may update signal-routes in RIB 140, which in turn triggers one or more routing protocols executed by routing protocol daemon 142 to advertise the updated signal-routes to routing protocol peer network devices, thereby causing network devices to route traffic differently and send network traffic requiring the affected services to a different services delivery gateway network device. Also in response to SRD 164 detecting the redundancy event, SRD 164 accordingly updates data specifying Stateful Sync roles, which may be stored in the service plane 134, for example. Stateful sync refers to session-level redundancy state maintained by on the service cards 136A, 136B of gateway 8 according to the stateful sync roles, which allows for synchronization of the necessary session-level state across master and standby service cards for a seamless transition from master to standby. This process is termed a switchover and ensures uninterrupted application services for the end user. By virtue of the message exchanges across all the members of a group, the techniques of this disclosure allow for continuous, fully-automated application switchovers across the chassis.

The user interface described herein also supports a syntax that provides the user the ability to define custom “dummy” redundancy events that can be used to manually pause switchovers or force switchovers, irrespective of the health status of applications.

As one example, a network address translation (NAT) function provided by one of service engines 152 may support a number of connections. Admin 145 may configure a threshold number of connections below which the NAT service should not fall for an expected performance level, and may use the syntax described herein to define a redundancy event (via UI module 146) that expresses the threshold number of connections. Admin 145 may also use the syntax described herein to define a redundancy policy (via UI module 146) that specifies an action to occur upon detection of the defined redundancy event, such as modifying a signal-route stored in RIB 140 to cause routing protocol daemon 142 to advertise an updated signal-route. Admin 145 can use the syntax described herein to further define one or more redundancy sets and redundancy groups. Management daemon 160 configures configuration database 162 to store the redundancy event, redundancy policy, redundancy sets, and redundancy groups. SRd 164 may continuously or periodically monitor the number of connections being supported by the NAT service, and SRD 164 detects that the number of connections available by the NAT service falls below the threshold number of connections, the SRd 164 detects occurrence of the redundancy event and applies the redundancy policy to trigger the designation of a new master for providing the NAT service, as described herein.

The techniques described herein decouple the application redundancy decision from the underlying network communication mechanism, using a protocol independent mechanism to communicate with the network layer. It allows for custom events to be triggered to simulate failure events to induce switchovers or switchbacks manually.

Further, as described herein, the techniques provide mechanisms by which applications are able to signal network protocols in a protocol agnostic manner using predesignated routes. The applications may use, for example, a route-signal vector, which is a predesignated set of routes called signal-routes each of which map to mastership and standby states which get updated by the application layer on the occurrence of predefined events such as critical faults and user-initiated transitions.

FIG. 4 is a block diagram illustrating an example set of service chains of services according to the techniques described herein. In one example approach, FIG. 4 illustrates a set of service chains 34A-34E supported by a service delivery gateway 8. Service chains 34 represent an example set of service chains provided by services 10 within, or external to, one or more service delivery gateways 8.

In this example, one or more subscriber packet flows 36A are directed along a first service chain 34A to receive network address translation (NAT) service 38. Similarly, one or more subscriber packet flows 36B are directed along a second service chain 34B for application of an HTTP filter service 40, NAT service 42 and session border controller (SBC) services 43 for voice over IP (VoIP) processing and control. In service chain 34C, packet flows 36C are directed only to HTTP filter service 44. In service chain 34D, packet flows 36D are directed to HTTP filter 46 and subsequently to firewall service 48. As another example, packet flows 36E are directed along service chain 34E for application of HTTP filter 50, NAT 52 and intrusion detection and prevention (e.g., deep packet inspection) service 54.

As noted above, current inter-chassis redundancy solutions are geared toward providing redundancy across two or more homogeneous chassis within the same network. The techniques disclosed herein provide a framework for application-aware inter-chassis redundancy with granular control to fail over groups of applications between sets of two or more network elements. The network elements can be homogeneous or heterogeneous (physical or virtual) and can spread across different networks or across geographies. The redundancy mechanism provides traffic redirection agnostic of underlying network protocols and provides options for triggering, preventing and resuming both manual and automated switchovers of groups of services based on their health status; the redundancy mechanism will be discussed further in the context of the discussion of FIG. 5 below.

FIG. 5 is a block diagram illustrating master and standby relationships across service delivery gateways 8 in a redundant service delivery system 27. System 27 shows redundancy sets 20 distributed across gateways 8A-8C. Service delivery gateways 8A-8C are connected via communications channel 26.

Each redundancy set in FIG. 5 is shown in either a Master state or a Standby state. The framework described above establishes a set of building blocks that provides the ability to extend service redundancy across multiple chassis for different groups, events and actions. The framework allows the user to define for each application the custom events that can be used as triggers for switchovers to other chassis and custom redundancy polices that include actions to be taken for the switchovers. The chassis that make up redundancy groups 22 can be homogeneous or heterogeneous chassis, they can be connected over either a L2 or a L3 networks and they can be geographically separated. In some examples, every redundancy set has a master and one or more standbys that get elected based on the health of the application associated with that redundancy set 20.

FIG. 5 also illustrates how two or more service delivery gateways 8 can operate in an “Active-Active” mode by subdividing the services mastership of particular redundancy sets 20 into multiple redundancy groups 22 distributed across the same set of chassis. For instance, referring to FIG. 4, health monitoring for network address translation (NAT) service 38, for intrusion detection and prevention (e.g., deep packet inspection) service 54 and for session border controller (SBC) services 43 may be assigned to redundancy set 3, while health monitoring for HTTP filter service 40 may be assigned to redundancy set 2. Since the mastership for both redundancy set 2 and 3 is shown to reside in the chassis for gateway 8B, all the above services are performed by gateway 8B. For this example, NAT service 38 executes on service card 136B of gateway 8B.

If NAT service 38 in gateway 8B were to experience a critical event (such as, e.g., failure of service card 136B), a redundancy event occurs for redundancy set 3, and mastership transitions to gateway 8C as shown in FIG. 5, transferring execution of all the services of redundancy set 3 to gateway 8C. This results in an “Active-Active” subdivision of services between gateways 8B and 8C. In this example, flows such as flows 36B, 36D and 36E will now be routed through both gateways 8B and 8C, while flow 36A may be routed only through gateway 8C and flow 36C remains within gateway 8B.

During operation, a service redundancy daemon 24 executing within each gateway 8 continuously monitors the health-status of groups of applications and exchanges this information across communications channel 26 to all chassis in the redundancy group. Each service redundancy daemon 24 also continuously monitors critical system and application faults. On the occurrence of such faults, depending on its mastership state, service redundancy daemon 24 communicates with the network layer in a protocol agnostic manner to redirect traffic to the next standby node slated to be elected as the master. This process is termed a switchover and it ensures uninterrupted application services for the end user. As detailed above, in one example approach, ICCP provides connectivity between peer redundancy groups 22. Such an approach is shown in more detail in FIG. 6.

FIG. 6 is a block diagram illustrating communication between gateways in the network system of FIG. 1. In the example shown in FIG. 6, redundancy groups communicate via an Inter-Chassis Control Process (such as a Inter-Chassis Control Protocol daemon (ICCPd) 30). In one such example approach, service redundancy daemon 164 establishes Unix (Interprocess Communication (IPC) with iccp for transport and notification service across communications channel 26. FIG. 6 also illustrates configuration information 32 used to configure ICCP.

In the example shown in FIG. 6, communications channels 26 between the gateways 8 that make up redundant service delivery system 27 exchange information between the gateways. In some such examples, Inter-Chassis Control Protocol (ICCP) provides connectivity to peer gateways 8. In one approach, ICCP is established as follows:

iccp {

-   -   local-ip-addr 1.1.1.1;     -   peer 2.2.2.2 {         -   redundancy-group-id-list 1;         -   liveness-detection {             -   minimum-interval 1000;         -   }     -   }

}

In some example approaches, Bidirectional Forwarding Detection (BFD) is used in addition to ICCP to detect failures. BFD 157 provides very fast failure detection in the forwarding plane 130 of gateway device 8. BFD 157 also provides a single mechanism for such detection independent of media, routing protocol and data protocol. In one example approach, BFD 157 executes in the packet forwarding engine of ingress forwarding component 150A. Such an approach ensures that the remote peer is transparent to control plane switchover if, for instance, non-stop forwarding is configured.

By virtue of the message exchanges across all the members of a group, the techniques described herein allow for continuous, fully-automated application switchovers across the chassis. They even provide the user the ability to pause switchovers or force switchovers, overriding the health status of applications. The techniques decouple the application redundancy decision from the underlying network communication mechanism, using a protocol independent mechanism to communicate with the network layer. It allows for custom events to be triggered to simulate failure events to induce switchovers or switchbacks manually.

In one example approach, management daemon 160 presents user interface module 146 by which an administrator 145 (“ADMIN”) can enter commands to configure gateway 8 and services engines 152. In some examples, user interface module 146 may be configured to receive text-based commands. According to the techniques of the invention, management daemon 160 supports a command syntax that allows administrator 145 to define redundancy events and routing policies that specify how gateway 8 is to respond to redundancy events. Management daemon 160 may store the configuration input received from administrator 145 as configuration data in configuration database 162, which may take the form of a text file, such as an ASCII file. Alternatively, management daemon 160 may process the configuration input and generate configuration data in any one of a number of forms, such as one or more databases, tables, data structures, or the like. Configuration data may take the form of one or more commands for adding new settings to the current configuration of gateway 8, commands for deleting or modifying existing settings of the current configuration, or combinations thereof. Gateway 8 may further parse configuration data and input from administrator 145, and resolve the references to appropriately configure gateway 8.

Specifically, administrator 145 inputs commands to user interface module 146 to configure routing policies for service redundancy daemon 164, as described in further detail below. Management daemon 160 then stores the routing policies in routing policy database 166.

In one example approach, administrator 145 may also input commands to user interface module 146 to configure other aspects of gateway 8. A services daemon in control unit 151 may, for instance, program service cards 136 with configuration data received from the administrator defining firewall zones and policies with respect to physical interfaces, causing the service cards 136 of services engines 152 to recognize the defined zones and applying the security policies when processing packets from data plane flow control unit 154.

As noted above, Redundancy Event (RE) is a critical event that triggers the SR daemon 164 to switch gateway 8 mastership to standby gateway 8. In one example approach, critical events include interface down events, FPC/PIC reboots, Routing Protocol daemon (RPd) aborts or restarts and Peer gateway events. In one example, each gateway 8 is configured via UI module 146 to monitor critical events selected by administrator 145 that cause a service delivery daemon 164 in one gateway 8 to release mastership and that lead a service delivery daemon 164 in another gateway 8 to take up mastership of the redundancy group.

In one example approach, administrator 145 defines a redundancy event RELS_MSHIP_CRIT_EV and lists the critical events that are members of that redundancy event. In one such example, the current configuration of redundancy event RELS_MSHIP_CRIT_EV may be displayed through a show command:

root@sdg1# show event-options redundancy-event RELS_MSHIP_CRIT_EV

and, in one example approach, the results displayed for redundancy-event RELS_(—) MSHIP_(—) CRIT_EV of gateway 8 are:

monitor {

-   -   link-down {         -   ae62.3203;         -   ams0.100;     -   ms-1/0/0;     -   }     -   process {         -   routing {             -   restart;             -   abort;         -   }     -   }

}

In the above example, a link-down event is triggered when, for instance, an interface down event occurs. A process event occurs when there is a Routing Protocol daemon 142 (RPd) restart. In this example, either the link-down event or the RPd restart event is sufficient to trigger a transfer of gateway mastership away from that gateway 8.

The above are simply examples of critical events. Other critical events may be defined as tied to specific services, or to service chains. For instance, a failure or degradation in a service provided by one or more service cards 136 could serve as a critical event, as could failure or degradation in one of the communication mechanisms available for communicating between gateways 8, or between a gateway 8 and another network device.

As noted above, a Redundancy Policy is a policy that ties a Redundancy Event to one or more actions to be taken on the occurrence of those critical events. In one example, an administrator can request the contents of redundancy policy REL_MSHIP_POL to be displayed through a show command as follows:

root@sdg1# show policy-options redundancy-policy RELS_MSHIP_POL

and the results are displayed for redundancy-policy RELS_MSHIP_POL of gateway 8 in one example as:

redundancy-event [RELS_MSHIP_CRIT_EV RELS_MSHIP_MANUAL_EV];

then {

-   -   release-mastership;     -   delete-static-route 10.45.45.0/24 {         -   routing-instance SGI-PRIVATE;     -   }

}

That is, if RELS_MSHIP_CRIT_EV is triggered by specified critical events, or administrator 145 triggers a mastership switch manually using redundant event RELS_MSHIP_MANUAL_EV, mastership is transferred to the highest rated standby gateway 8.

In another example, an administrator can request the contents of redundancy policy ACQU_MSHIP_POL to be displayed through a show command as follows:

root@sdg1# show policy-options redundancy-policy ACQU_MSHIP_POL

and the results are displayed for redundancy-policy ACQU_MSHIP_POL of gateway 8 in one example as:

redundancy-event ACQU_MSHIP_CRIT_EV;

then {

-   -   acquire-mastership;     -   add-static-route 10.45.45.0/24 {         -   receive;         -   routing-instance SGI-PRIVATE;     -   }

}

In one example approach, an administrator can request the contents of redundancy policy WARN_POL to be displayed through a show command as follows:

root@sdg1# show policy-options redundancy-policy ACQU_MSHIP_POL

and the results are displayed for redundancy-policy ACQU_MSHIP_POL of gateway 8 in one example as:

redundancy-events WARN_EV;

then {

-   -   broadcast-warning;

}

One approach for setting up gateway 8 to trigger a mastership change is as follows:

root@sdg1>request services redundancy-set id 1 trigger redundancy-event ACQU_MSHIP_MANUAL_EV {force mastership acquisition}

root@sdg1>request services redundancy-set id 1 trigger redundancy-event RELS_MSHIP_MANUAL_EV {force mastership release}

As noted above, a redundancy set establishes the granularity of the master/standby states driven by redundancy policies. In one example approach, a redundancy set may also bind one or more service-sets to drive the Stateful Sync state related to the service-sets. In one example, a first redundancy set (redundancy-set) is defined for a gateway 8 and can be displayed through a show command as follows:

root@sdg1# show services redundancy-set

and the results are displayed for one example redundancy set as:

traceoptions {

-   -   level all;     -   flag all;

}

1 {

-   -   redundancy-group 1;     -   redundancy-policy [ACQU_MSHIP_POL RELS_MSHIP_POL];     -   hold-time 10;

}

root@sdg# show services service-set CGN4_SP-7-0-0

replicate-services {

-   -   replication-threshold 360;     -   stateful-firewall;     -   nat;

}

+redundancy-set 1;

A Redundancy Group (RG) is a collection of Redundancy Sets and defines common peering properties across a set of gateways 8. RGs allow for different peering settings across same peers. In one example, a first redundancy group (redundancy-group 1) is defined for a gateway 8 and can be displayed through the same show command as used for the redundancy set, and achieves the same result:

-   -   root@sdg1# show services redundancy-group         and the results are displayed for one example redundancy group 1         as:

traceoptions {

-   -   level all;     -   flag all;

}

1 {

-   -   redundancy-group 1;     -   redundancy-policy [ACQU_MSHIP_POL RELS_MSHIP_POL];     -   hold-time 10;

}

where, in some examples, hold-time is a delay taken before implementing the redundancy policy.

FIG. 7 is a block diagram illustrating a mastership transition to a peer in accordance with techniques described herein. In some examples, Peer Events are defined as a type of Redundancy Events that are exchanged between SRd peers. Redundancy Policies tied to Peer Events allow a local SRd peer to take action based on Peer Events reported by remote SRd peers as shown in FIG. 7. In the example of FIG. 7, a redundancy event occurs on service delivery gateway 8A, and SDG1 releases mastership to SDG2 executing on service delivery gateway 8B. In one example approach, a message is sent to SFG2 from SDG1 via ICCP telling SDG2 to take over as master. In one such example, a group of peer events are defined for a gateway 8. In some such examples, the members of the group of critical peer events can be displayed through a show command as follows:

root@sdg2# show event-options redundancy-events PEER_MSHIP_RELS_EV

and the results are displayed as:

monitor {

-   -   peer {         -   mastership-release;     -   }

}

and

root@sdg2# show event-options redundancy-event PEER_MSHIP_ACQU_EV

monitor {

-   -   peer {         -   mastership-acquire;     -   }

}

root@sdg2# show event-options redundancy-events WARN_EV

monitor {

-   -   link-down {         -   ae62.3203;         -   ams0.100;         -   ms-1/0/0;     -   }     -   process {         -   routing {             -   restart;             -   abort;         -   }     -   }..

One potential advantage of the user interface and redundancy framework discussed above may be the ability to provide redundancy for one or more groups of applications across two or more chassis, independent of the underlying network protocols. The framework is highly extensible by the use of the system of redundancy events, redundancy policies, and redundancy groups, making is easy to incorporate new events and actions for supporting network function virtualization (NFV) based use-cases. The redundancy framework may provide for uninterrupted availability of services (termed Non-Stop Services for applications). The redundancy framework also allows a set of two or more chassis function in an ‘Active-Active’ mode by virtue of sub dividing the services mastership in to multiple redundancy-groups across the same set of chassis. The approach described herein is also routing protocol agnostic, in that the syntax allows the administrator to express the redundancy events and redundancy policies in a way that is independent of the underlying routing protocol that is ultimately used by the router for communicating a change in mastership.

In one example approach, SRd 164 may also be used to synchronize the configuration across all the peers in a redundancy set. Consider the following Carrier Grade Network Address Translation (CGNAT) configuration stanza (tied to a redundancy set, RS #1),

service-set SFW6_AMS0 {

-   -   syslog {         -   host 1.2.1.1 {             -   services any;             -   class {                 -   session-logs;                 -   stateful-firewall-logs;                 -   alg-logs;             -   }             -   source-address 1.2.1.2;             -   }         -   }         -   stateful-firewall-rules SFW6_AMS0-r1;         -   next-hop-service {             -   inside-service-interface ams0.199;             -   outside-service-interface ams0.200;         -   }         -   redundancy-set-id 1;     -   }         through which SRd 164 can ensure that contents of service-set         CGN4_AMS0, which is tied to RS 1, are synchronized all the nodes         that are a part of RS 1.

Any event in a router or gateway can be mapped into a redundancy event and used to trigger a transition in the state of a redundancy set. In one example approach, a generic operating system event (e.g., a generic system JUNOS event) is configured as a redundancy event (RE) as follows:

policy EVNTD_TO_SRD_POL {   events CHASSISD_SNMP_TRAP10; ← Generic JUNOS event       attributes-match {   chassisd_snmp_trap10.trap matches “FRU power off”;   }   then { +    trigger-redundancy-event RELS_MSHIP_CRIT_EV ← Converted to       RE +     arguments { +        fru_slot “ {$$.value7}”; +      }    }  } }

FIG. 8A is a block diagram illustrating communication between application nodes in the redundant service delivery gateway system of FIG. 1. In the example of FIG. 8A, three network elements (network nodes 60A-60C) communicate through communications channel 26. Application services nodes 62A through 62C are associated with network elements 60A through 60C, respectively, and are arranged in a master/standby relationship. In one example approach, the communication mechanism shown in FIG. 8A and described herein permits communicating in a protocol and network agnostic manner between application nodes in redundant service delivery system 27.

In one example, two or more network elements 36 make our application overlay network of which a single node 62 (here, 62A) is the application (services) master and one or more nodes 62 are standby. Such an application network is said to be resilient when, on the occurrence of a critical fault on the master, the next standby in the hierarchy can automatically take over the mastership, ensuring uninterrupted application services. In one example methodology, application services nodes 62 signal network-protocols in a protocol agnostic manner using predesignated routes.

In one example approach, a route-signal vector 70 is used to store state for each redundancy set. As can be seen in the example shown in FIG. 8B, route-signal vector 70 is a predesignated set of routes called signal-routes each of which map to mastership and standby states. In the example shown in FIG. 8B, route-signal vector 70 includes one or more signal-routes 72 and one or more signal-route states 74 organized as signal-route/signal-route state pairs. In some example approaches, state 74 is a zero when the redundancy set associated with the signal route 72 is to be in a stand-by state and a one when the signal route 72 is to be in a master state. In other example approaches, state 74 is a zero when the redundancy set associated with the signal route 72 is to be in a master state and a one when the signal route 72 is to be in a stand-by state.

In some approaches, an application executing in the application layer updates the signal-routes on the occurrence of redundancy events such as critical faults. It is not a requirement for the destination represented by the routes that make up the route-signal vector 70 to be reachable since they are used only for signaling between the application and the network layers. Routing-policies, which drive routing-protocols, are coupled to the existence or non-existence of these routes. As soon as the application layer (e.g., SRd 164) adds or removes these signal-routes from RIB 140, the routing policies are implicated, resulting in in the next most preferred standby taking application mastership. Standby nodes synchronize state from the master so that anytime the master fails, the next best standby can take over the mastership. An application services node 62 taking over application mastership begins to perform one or more application services such as Mobility services, Stateful firewall, CGNAT, NAT, IDP, proxy services, application acceleration, etc., on the network traffic. The added bonus of this methodology is its ability to ensure resiliency over L2 networks, apart from L3 networks, by communicating with L2 protocols such as Virtual Router Redundancy Protocol (VRRP) which support route-tracking. In fact, since SRd 24 interacts with L3, L2 and Stateful Sync components of router operating systems, respective debugging commands can be used to troubleshoot SRd 24 interaction. SRD 24 may also generate Simple Network Management Protocol (SNMP) traps for state changes. SNMP traps may be used, for example, to notify another device of a change in state of one of an application executing on one of the redundant service delivery gateways. SNMP traps do this by sending a message known as a trap of the event to the other device.

In one example approach, an if-route-exists condition detects the presence or absence of a signal-route. In one such example approach, a change in the presence or absence of a particular signal-route is advertised using as-path-prepend values. In one such example approach, a change in the presence or absence of a particular signal-route is advertised using different local-preference values.

Another potential advantage of this methodology is the ability for applications to drive resiliency of the application overlay network over both L2 and L3 networks in a protocol-agnostic manner. The techniques described may allow for applications to create an overlay network of peers and allows for applications 62 to drive and adapt the routing over the underlying network. It also allows for a geo-redundant inter-chassis redundancy solution. In a real world application, the approach reduced route convergence time by 95% (convergence was in about a second), ensuring uninterrupted failovers for millions of application sessions across the overlay network for a 99.999% High Availability.

In one example, the signal-routes are static routes manipulated by SRd 24 based on the mastership state changes. An example of adding a static route is:

root@sdg2# show policy-options redundancy-policy ACQU_MSHIP_POL

and the result is:

redundancy-events PEER_MSHIP_RELS_EV;

then {

-   -   acquire-mastership;     -   add-static-route 10.45.45.0/24 {         -   receive;         -   routing-instance SGI-PRIVATE;     -   }

}

where routing protocol daemon 142 adds a new static route at 10.45.45.0/24.

In some example approaches, routing policies advertise routes based on the existence or non-existence of signal-routes. In one such example approach, routing policies are preconfigured to advertise routes based on the existence or non-existence of signal-routes using the if-route-exists condition. For example,

policy-options {

condition ROUTE_EXISTS{

-   -   if-route-exists {         -   10.45.45.0/24;         -   table SGI-PRIVATE.inet.0;     -   }

}

}

and

policy-statement BGP-EXPORT-DEF-V6-ONLY {

term 1 {

-   -   from {         -   prefix-list default-route-v6;         -   condition ROUTE_EXISTS;     -   }     -   then {         -   as-path-prepend “64674 64674 64674 64674”;         -   next-hop self;         -   accept;     -   }

}

}

In this example, a check is made in the prefix-list to determine if the route 10.45.45.0/24 exists and, if the route is not present, SRd 24 increases the cost of the route using the as-path-prepend command. The Autonomous System (AS) path prepend can be used to tell other routing entities in system 4 to route to a gateway device having a lower routing cost, which also is the gateway device next in line to assume mastership. BGP prefers the shortest AS path to reach a destination. The path that BGP will choose to reach a destination can be manipulated using AS path prepending. AS path prepending allows for artificially lengthening the AS path that BGP advertises to a neighbor. An example as-path-prepend approach is further illustrated in FIG. 9.

In another example approach, as shown in FIG. 10, the presence or absence of the signal-route is advertised through the use of local-preference values. A local-preference value is a metric used by BGP sessions to indicate the degree of preference for an external route. The route with the highest local preference value is preferred. The local-preference attribute is used in inbound routing policy and is advertised to internal BGP peers and to neighboring confederations. In one such approach, a service delivery gateway master defaults its local-preference value to, for example, 400, while the standby defaults to 350. If mastership transitions to the standby gateway, its local-preference value may be, for example, raised to 450, while the former master retains its local-preference value of 400.

In some examples, SRd 24 drives L2 connectivity via VRRP route tracking. VRRP is a layer-2 (switching) protocol unlike the routing protocols explained earlier. VRRP route tracking is a VRRP feature which tracks the reachability of signal-routes in order to vary the VRRP priorities dynamically. In one example approach, VRRP route tracking is used to help advertise the route switching signaling the change of mastership. In one such example, VRRP route tracking is configured as follows:

interfaces {  ge-1/0/1 {   unit 0 {    vlan-id 1;     family inet {      address 200.100.50.1/24 {      vrrp-group 0 {      virtual-address 200.100.50.101;      priority 200;      track {       route 10.45.45.0/24 ;       table SGI-PRIVATE.inet.0;      }     }. . .

FIG. 11 is a flowchart illustrating services switchover to a peer in accordance with techniques described herein. In the example shown in FIG. 11, SRd 164 assumes mastership (200) and begins to monitor for critical events as defined by the redundancy events configuration (202). If SRd 164 detects that a critical event occurs, SRd 164 adds or removes a route from the RIB based on a relevant route policy (204). In one example approach (e.g., where SRd 164 is executing on a gateway that is no longer the master), SRd 164 also notifies the preferred standby gateway 8 that it is to take over mastership. In one such example approach, SRd 164 notifies SRd 164 of the preferred standby gateway 8A using ICCP.

Gateway 8 then advertises the change in routes (206). In one example approach, VRRP is used to communicate the advertised priorities for one or more routes. Routers in the network receive the advertisements and, based on the advertised routes, begin forwarding network traffic to the next preferred standby gateway 8 for application of services (208).

In one example approach, each SR daemon 164 on the routing engine continuously monitors preconfigured redundancy events. On the occurrence of a redundancy event, SRd 164 a) adds or removes signal-routes from RIB 140 as specified in the redundancy policy and b) updates Stateful Sync roles accordingly. Stateful sync refers to session-level redundancy provided on the service cards 136A, 136B of gateway 8. In one such approach, each service 10 maintains its own application state and shares that application state with its counterparts in standby gateways 8. In some examples, SRd 164 maintains the application state associated with each of the services 10 and shares the application state.

In one example approach, the addition or removal of signal-routes by SRd 164 causes routing protocol daemon 142 to send a routing protocol update message to routing protocol peers to advertise the changes in the routes. In one such example approach, this involves changing the cost of the route via, for example, the as-path-prepend command discussed above.

In some example approaches, the route change also has an effect on the VRRP configuration tracking this route, resulting in different VRRP priority advertisements as noted above. The newly advertised routes and changed VRRP priorities redirect traffic to the next preferred standby gateway 8, and SRd 164 switches over the services mastership to that gateway 8. In some such example approaches, VRRP is also used to communicate, to a SRd 164 in another gateway 8, the need for a mastership transition.

FIG. 12 is a block diagram illustrating a redundancy set state machine for moving between master and standby states in accordance to the techniques described herein. The state machine of FIG. 12 shows the states of an instance of service redundancy daemon 24 of a gateway 8 in system 27. As can be seen in FIG. 12, SRd 164 boots into an Init state (220) and remains there until SRd 164 receives a health check success or failure. In one example approach, a gateway assigned three redundancy sets maintains three instances of the state machine shown in FIG. 12. In one such example approach, each state machine operates independently of the other.

On a health check success, control moves to a Standby Ready state (222), and remains there until a critical event occurs or a mastership event occurs. On a health check failure, control moves to a Standby Warning state (224), and remains there until a health check success occurs or a forced mastership event occurs.

On a critical event, control moves from state 222 to a Standby Warned state 224. On a mastership event, control moves from state 222 to a Master state 226.

Once in the Master state 226, gateway 8 remains in the Master state (226) until a critical event occurs. If a critical event occurs, gateway 8 moves to a Standby Warned state (224).

Once in the Standby Warned state (224), gateway 8 remains in that state until a forced mastership acquire event forces it back to the Master state (226), or a health check success moves it back into the Standby Ready state (222). In some example approaches, SRd 164 sets a timer on gateway 8 entering Standby Warned state (224). When the timer times out, SRd 164 checks to determine if gateway 8 has recovered from the event. If so, control moves back to Standby Ready state (222). In some such example approaches, control remains in Standby Warned state 224 until it is initialized or has received a health check success. In some such example approaches, a check is made each time the timer times out to see if the critical event has been resolved.

Other standby states may be added to reflect intermediate standby states.

The techniques described above may offer advantages over previous approaches to redundancy between gateways. SR daemon 164 on routing component 138 continuously monitors preconfigured Redundancy Events. On the occurrence of Redundancy Events, SR daemon 164 adds or removes Signal-routes specified in the Redundancy Policy and updates Stateful Sync roles appropriately. The resulting route change affects the routing policy connected to this route and causes routing protocols executing on the gateways to advertise routes differently. If VRRP is being used, VRRP configuration tracking of this route results in different VRRP priority advertisements. The newly advertised routes and VRRP priorities cause routing peers to redirect traffic to the standby gateway 8 and SRd 164 switches over the services mastership to the standby gateway 8. In one example approach, as noted above, VRRP may also be used to notify a SRd 164 on another gateway 8 that it is to take over mastership of the redundancy set.

The techniques described herein may be implemented in hardware, software, firmware, or any combination thereof. Various features described as modules, units or components may be implemented together in an integrated logic device or separately as discrete but interoperable logic devices or other hardware devices. In some cases, various features of electronic circuitry may be implemented as one or more integrated circuit devices, such as an integrated circuit chip or chipset.

If implemented in hardware, this disclosure may be directed to an apparatus such as a processor or an integrated circuit device, such as an integrated circuit chip or chipset. Alternatively or additionally, if implemented in software or firmware, the techniques may be realized at least in part by a computer-readable data storage medium comprising instructions that, when executed, cause a processor to perform one or more of the methods described above. For example, the computer-readable data storage medium may store such instructions for execution by a processor.

A computer-readable medium may form part of a computer program product, which may include packaging materials. A computer-readable medium may comprise a computer data storage medium such as random access memory (RAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), Flash memory, magnetic or optical data storage media, and the like. In some examples, an article of manufacture may comprise one or more computer-readable storage media.

In some examples, the computer-readable storage media may comprise non-transitory media. The term “non-transitory” may indicate that the storage medium is not embodied in a carrier wave or a propagated signal. In certain examples, a non-transitory storage medium may store data that can, over time, change (e.g., in RAM or cache).

The code or instructions may be software and/or firmware executed by processing circuitry including one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, functionality described in this disclosure may be provided within software modules or hardware modules.

Various example approaches have been described. These and other approaches are within the scope of the following claims. 

What is claimed is:
 1. A method, comprising: receiving configuration information defining events that cause a transition from a master redundancy state to a standby redundancy state in service delivery gateways, including receiving configuration information defining a first event that causes a transition from a first master redundancy state to a first standby redundancy state in service delivery gateways; in response to receiving the configuration information, storing a plurality of signal-routes, including a first signal-route, wherein each signal-route is a route used by applications to signal changes in application state and wherein each signal-route is associated with one or more of the defined events; and in response to detecting occurrence of the first event in a first service delivery gateway: transitioning, within the first service delivery gateway, from the first master redundancy state to the first standby redundancy state; making a change in a set of available signal-routes stored in the first service delivery gateway, wherein making a change in a set of available signal-routes includes adding the first signal-route to or removing the first signal-route from the set of available signal routes in the first service delivery gateway; and advertising, from the first service delivery gateway and to peer network devices, the change in the first signal-route.
 2. The method of claim 1, wherein advertising the change in the first signal-route includes sending a route update message from a routing protocol process executing on one or more of the service delivery gateways in response to a message from a services redundancy process executing in the first service delivery gateway.
 3. The method of claim 2, wherein the route update message includes a route cost attribute, wherein the route cost attribute indicates that a route to the first service delivery gateway associated with the first signal-route is higher cost than a corresponding route to the second service delivery gateway.
 4. The method of claim 1, wherein the service delivery gateways include a second service delivery gateway and wherein the method further comprises: transitioning, within the second service delivery gateway, from the first standby redundancy state to the first master redundancy state in response to detecting occurrence of the first event in the first service delivery gateway.
 5. The method of claim 4, wherein each service delivery gateway includes an application layer, and wherein making a change in the first signal-route from the set of available signal-routes occurs at the application layer and causes a change in routing policy within the first service delivery gateway.
 6. The method of claim 5, wherein each signal-route is a static route and wherein the first event is a critical event associated with a service provided by the first and second service delivery gateways.
 7. The method of claim 1, wherein each signal-route is a static route, wherein each service delivery gateway includes an application layer and wherein adding the first signal-route to or removing the first signal-route from the set of available signal-routes occurs at the application layer.
 8. The method of claim 1, wherein advertising the change in the first signal-route includes advertising a different Virtual Router Redundancy Protocol (VRRP) priority for the first signal-route.
 9. The method of claim 1, wherein each service delivery gateway is configured to support one or more redundancy sets, including a first redundancy set, wherein each redundancy set is associated with one or more of the defined events, wherein each of the defined events triggers a transition from master redundancy state to standby redundancy state in a state machine of the redundancy set associated with the defined event, wherein each defined event is associated with a redundancy policy that establishes an action to be taken on occurrence of the defined event, and wherein transitioning, within the first service delivery gateway, from the first master redundancy state to the first standby redundancy state in response to detecting the first event includes establishing a services redundancy process, monitoring for defined events with the services redundancy process, and initiating, within the services redundancy process, a transition from the first master redundancy state to the first standby redundancy state in the first service delivery gateway on occurrence of the first event.
 10. The method of claim 9, wherein advertising a change in the first signal-route includes advertising a different Virtual Router Redundancy Protocol (VRRP) priority for the first signal-route.
 11. The method of claim 9, wherein advertising a change in the first signal-route includes sending a route update message from a routing protocol process.
 12. A service delivery gateway system, comprising: a plurality of links; and a plurality of service delivery gateway network devices connected by the plurality of links, wherein the plurality of service delivery gateway network devices includes a first service delivery gateway network device and a second service delivery gateway network device, wherein each service delivery gateway network device includes a memory and one or more processors connected to the memory, wherein the one or more processors are configured to: receive configuration information defining events that cause a transition from a master redundancy state to a standby redundancy state in service delivery gateway network devices, including receiving configuration information defining a first event that causes a transition from a first master redundancy state to a first standby redundancy state in service delivery gateway network devices; in response to receiving the configuration information, store a plurality of signal-routes, including a first signal-route, wherein each signal-route is a route used by applications to signal changes in application state and wherein each signal-route is associated with one or more of the defined events; and in response to detecting occurrence of the first event in the first service delivery gateway network device: transition, within the first service delivery gateway network device, from the first master redundancy state to the first standby redundancy state; transition, within the second service delivery gateway, from the first standby redundancy state to the first master redundancy state; make a change in a set of available signal-routes stored in the first service delivery gateway, wherein making a change in a set of available signal-routes includes adding the first signal-route to or removing the first signal-route from the set of available signal routes in the first service delivery gateway; and advertise, from the first service delivery gateway and to peer network devices, the change in the first signal-route.
 13. The system of claim 12, wherein a process executing in an application layer makes the change in the first signal-route by adding or removing, at the application layer, the first signal-route from a routing information base.
 14. The system of claim 13, wherein the service delivery gateway network device advertises the change in the first signal-route by advertising a different Virtual Router Redundancy Protocol (VRRP) priority for the first signal-route.
 15. The system of claim 13, wherein the service delivery gateway network device advertises the change in the first signal-route by sending a route update message from a routing protocol process in response to a message from a services redundancy process executing in the first service delivery gateway network device.
 16. The system of claim 15, wherein the route update message includes a route cost attribute, wherein the route cost attribute indicates that a route to the first service delivery gateway network device associated with the first signal-route is higher cost than a corresponding route to the second service delivery gateway network device.
 17. The system of claim 12, wherein a services redundancy process executing in the first service delivery gateway network device makes a change in the first signal-route via a message to a routing protocol process.
 18. The system of claim 12, wherein each signal-route is a static route and wherein the first event is a critical event associated with a service provided by the first and second service delivery gateways network devices.
 19. The system of claim 12, wherein each service delivery gateway network device further includes a communication interface and a user interface, wherein the user interface configures each service delivery gateway network device as one or more redundancy sets, including a first redundancy set, via the configuration information, wherein each redundancy set is associated with one or more of the defined events, wherein each of the defined events triggers a transition from master redundancy state to standby redundancy state in a state machine of the redundancy set associated with the defined event, wherein each defined event is associated with a redundancy policy that establishes an action to be taken on occurrence of the defined event, and wherein the processors are configured, via the configuration information, to transition, within the first service delivery gateway network device, from the first master redundancy state to the first standby redundancy state in response to detecting the first event by establishing a services redundancy process, monitoring for defined events with the services redundancy process, and initiating a transition from the first master redundancy state to the first standby redundancy state in the first service delivery gateway on occurrence of the first event.
 20. The system of claim 19, wherein the services redundancy communicates the transition via a Virtual Router Redundancy Protocol (VRRP) message.
 21. The system of claim 19, wherein advertising a change in the first signal-route includes sending a route update message from a routing protocol process.
 22. A non-transitory computer-readable storage medium storing instructions that, when executed, cause one or more processors to: receive configuration information defining events that cause a transition from a master redundancy state to a standby redundancy state in service delivery gateway network devices, including receiving configuration information defining a first event that causes a transition from a first master redundancy state to a first standby redundancy state in service delivery gateway network devices; in response to receiving the configuration information, store a plurality of signal-routes, including a first signal-route, wherein each signal-route is a route used by applications to signal changes in application state and wherein each signal-route is associated with one or more of the defined events; and in response to detecting occurrence of the first event in a first service delivery gateway network device: transition, within the first service delivery gateway network device, from the first master redundancy state to the first standby redundancy state; make a change in a set of available signal-routes stored in the first service delivery gateway, wherein making a change in a set of available signal-routes includes adding the first signal-route to or removing the first signal-route from the set of available signal routes in the first service delivery gateway; and advertise, from the first service delivery gateway and to peer network devices, the change in the first signal-route. 